It is my opinion that a player should be given a subforum if they fullfiled these two criteria:

a) have a large fan base (cause those are the people for whom these subforums are opened in the first place)
b) maintain their position inside top 50 for an extended period of time, let's say 18 months

The first one is self explanatory and the second suggests that we don't open subforums for players who have made one (lucky) streak and bombed right back down the rankings as soon as their points from this one (or two) event(s) got deducted a year later.